Evidence exists from elsewhere and from our Laboratory that human brain tumors are composed of heterogeneous cell populations that account for their pleomorphism, biological change toward increased malignancy over time, and the occurrence of mixed gliomas and gliosarcomas. Such heterogeneity is likely responsible for the relatively poor results achieved in the chemotherapy of patients harboring malignant gliomas. We have demonstrated the presence of such heterogeneity by cloning out these heterogeneous cell populations in tissue cultures from gliomas obtained from patients undergoing resections of their tumors. The clonal lines have been characterized karyotypically and by morphology, glial fibrillary acidic protein, growth and chemosensitivity in vitro. In addition, the lines are tested by inoculation into athymic nude mice to determine in vivo morphology, growth kinetics and chemosensivity. These studies continue, the clonal lines being co-cultivated to determine if phenotypic expression for drug sensitivity is altered and, if so, whether by intermonoclonal antibody formation is under investigation as is cell surface markers by 2D electrophoresis.